package com.test.io;

import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;

public class LockingMappedfiles {
	static final int LENGTH = 0x8ffffff;//128MB
	static FileChannel fc;
	public static void main(String[] args)throws Exception{
		fc = new RandomAccessFile("test.dat", "rw").getChannel();
		MappedByteBuffer out = fc.map(FileChannel.MapMode.READ_WRITE, 0, LENGTH);
		for(int i=0; i<LENGTH;i++){
			out.put((byte)'x');
			new LockAndModify(out, 0, 0+LENGTH/3);
			new LockAndModify(out, LENGTH/2,LENGTH/2+LENGTH/4 );
		}
	}
	private static class LockAndModify extends Thread{
		private ByteBuffer buff;
		private int start,end;
		public LockAndModify(ByteBuffer mbb, int start , int end) {
			this.start = start;
			this.end = end;
			mbb.limit(end);
			mbb.position(start);
			buff = mbb.slice();
			start();
		}
		@Override
		public void run() {
			try {
				//exclusive lock with no overlap:
				java.nio.channels.FileLock fl = fc.lock(start, end, false);
				System.out.println("Locked: " +start +" to "+end);
				//perform modification
				while(buff.position()<buff.limit()-1){
					buff.put((byte)(buff.get()+1));
				}
				fl.release();
				System.out.println("Released: " +start+" to "+end);
			} catch (Exception e) {
				e.printStackTrace();
			}
		}
		
	}
}
